The growth of electropositive metal films from chemical precursors is extremely challenging due to the difficult reductions of the precursor metal ions and the lack of powerful reducing co-reagents that can transform the metal ions to the metals rapidly.1,2 A significant advance in the past 10 years has been the development of atomic layer deposition (ALD) film growth processes for noble metals (Ru, Os, Rh, Ir, Pd, Pt) that entail a metalorganic metal precursor and an oxygen source (O2, O3) at temperatures of >200° C.1d The oxygen source leads to combustion of the ligands of the metal precursor, yielding an intermediate metal oxide, which then decomposes to the metal at >200° C. The key to success in these depositions is the positive electrochemical potentials of the noble metal ions (E°>0 V), which allow the oxide ion to reduce the metal ions to the metals. By contrast, most other metal ions in the periodic table have negative E° values and are much more difficult to reduce to the metals.1 As such, the thermal ALD growth of these metals is difficult and remains poorly developed.1 H2 has been the most commonly used reducing co-reagent to date in ALD,1,3 but many metal ions have low reactivities toward H2 at desired ALD growth temperatures of ≤250° C.1 
BH3(NHMe2) serves as a powerful reducing co-reagent for Ni(II), Co(II), Fe(II), Cr(II), and possibly Mn(II) α-imino alkoxide precursors in the ALD growth of these metals at ≤200° C.2 However, these processes required a Ru substrate to decompose the BH3(NHMe2) to more reactive reducing species, and the growth stopped once the film covered the Ru surface. The use of 1,4-bis(trimethylsilyl)-2-methyl-2,5-cyclohexadiene4 (1) and 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine5 (2) was disclosed as precursors for the ALD growth of titanium metal and other electropositive films.6 Precursors 1 and 2 likely react with metal halide precursors through elimination of ClSiMe3 and formation of intermediate 1,4-metalla-2-methyl-2,5-cyclohexadienyl (from 1) or 1,4-metalla-1,4-dihydropyrazinyl (from 2) complexes, which then eliminate aromatic toluene or pyrazine and thereby reduce the metal centers by 2 electrons per ligand. Precursors 1 and 2 have allowed the first thermal ALD growth of titanium metal films (Ti(II)↔Ti(O), E°=−1.631 V),6c and should also lead to the ALD growth of many other electropositive metal films. However, precursors 1 and 2 likely require metal halide precursors, many of which are corrosive. Additionally, ClSiMe3 is produced in the deposition chemistry (FIG. 1), which may also be corrosive toward metal films.

Accordingly, there is a need for improved reducing agents for ALD and CVD processes.